A digital video cassette recorder employing a helical scanning format has been proposed by a standardization committee. The proposed standard specifies digital recording of either standard definition (SD) television signals, for example NTSC or PAL, and high definition television signals having an MPEG compatible structure, such as a proposed Grand Alliance signal. The SD recorder utilizes a compressed component video signal format employing intra field/frame DCT with adaptive quantization and variable Length coding. The SD track format comprises 10 μm tracks, azimuth recorded without guard bands, with 10 or 12 tracks per NTSC or PAL frame respectively. The tape cassette employs ¼″ wide tape with an evaporated metal recording medium. The SD digital VCR or DVCR, is intended for consumer use and has sufficient data recording capability to record either NTSC (PAL) signals, or an advanced television signal.
An advanced television or ATV signal has been developed by the Grand Alliance (GA) consortium. A specification document titled Grand Alliance HDTV System Specification was published in the 1994 Proceeding of the 48th Annual Broadcast Engineering Conference Proceedings. The GA signal employs an MPEG compatible coding method which utilizes an intra-frame coded picture, termed I frame, a forward predicted frame, termed a P frame and a bidirectionally predicted frame, termed a B frame. These three types of frames occur in a group known as a GOP or Group Of Pictures. The number of frames in a GOP is user definable but may comprise, for example, 15 frames. Each GOP contains one I frame, which is abutted by B frames, which are then interleaved with P frames.
In an analog consumer VCR, “Trick Play” or TP features such as picture in forward or reverse shuttle, fast or slow motion, are readily achievable, since each recorded track typically contains one field. Hence reproduction at speeds other than standard, result in the reproducing head, or heads, crossing multiple tracks, and recovering recognizable horizontal picture segments. The GOP of an ATV signal, employing I, P and B frames, may be recorded occupying multiple tracks on tape, for example, 10 tracks per frame and 150 tracks per GOP. Simply stated, when a DVCR is operated at a non-standard reproduction speed, replay heads transduce sections or segments from multiple tracks. Unfortunately these track segments no longer represent sections from discrete records of consecutive image fields. Instead, the segments contain data resulting mainly from predicted frames of the GOP. During play speed operation, I frame data is recovered which permits the reconstruction of the predicted B and P frames. Clearly, during “Trick Play” operation, the amount of I frame data recovered progressively diminishes as TP speed increases. Hence, the possibility of reconstructing B and P frames from the reproduced pieces of I frame data is virtually zero. Thus, the provision of “Trick Play” or non-standard speed replay features requires that specific data be recorded, which when reproduced in a TP mode, is capable of image reconstruction without the use of adjacent frame information. Furthermore, since “Trick Play” specific data is recorded, the physical track location must be such to permit recovery in a TP mode.